1. Field of the Invention
The present invention relates to methods for treating and painting the surface of a molded article made of a thermoplastic elastomer composition. More particularly, the present invention relates to methods for treating and painting the surface of a molded article made of a thermoplastic elastomer composition derived from a mixture of an oil-extended olefinic copolymer rubber, an olefinic plastic and a bismaleimide compound in a specific proportion.
2. Description of the Prior Art
Thermoplastic elastomers are already known materials and are steadily finding in recent years increased applications chiefly in the industrial field of motor vehicles and domestic electric appliances, utilizing their non-rigid soft property and such a characteristic that they are moldable by way of extrusion molding or injection molding utilizable for conventional thermoplastic resins.
Among these thermoplastic elastomers, olefinic thermoplastic elastomers possess, in addition to the above mentioned beneficial properties, a low specific gravity, heat-resisting property and a well balance of other physical properties. Accordingly, they are especially useful in the field of auto parts with the tendency of rendering automobiles lightweight. In recent years, they begin to be utilized even as bumpers of automobiles where a large scale injection molding technique is demanded. In bumpers as exterior parts, a mood is seen to make them fashionable by applying a paint thereonto in combination with the body color.
On the other hand, printing of patterns is frequently applied also to domestic electric appliances to enhence their ornamental value.
From the past, compositions comprising olefinic copolymer rubbers and olefinic plastic substances are known as the olefinic thermoplastic elastomers. As they are devoid of polarity, however, a surface treatment for enhancing adhesiveness or paintability of the elastomers is often difficult. In order to improve these properties, a single layer of a primer comprising, for example, a chlorinated polypropylene is generally interposed between the olefinic thermoplastic elasomer and an adhesive or a paint thereby enhancing compatibility between the paint and the elastomer. However, such primer itself is expensive and one step for forming a primer layer is added to the treatment, thus making the cost of the final product higher. Accordingly, an improvement is desired in this respect.
In order to improve adhesiveness of the elastomers, for example, a surface treatment including modification of the surface by etching in a physical or chemical manner is discussed and actually applied to shaped articles made of polypropylene. In this method, however, the effect of improvement is insufficient and some problems are raised in that the treatment is complicated rather than the step for coating a primer and that deformation of the shaped article may take place depending on the condition of etching.
Further, a method for improving paintability is now being adopted wherein the surface of shaped articles is oxidatively polarized by plasma irradiation in place of coating a primer. However, a problem is also involved in this method; the step for plasma irradiation is batchwise and the treatment condition is not even depending on the shape of the articles. At present, therefore, a satisfactory result is not achieved in connection with imparting polarity to the surface of the olefinic thermoplastic elastomers. Accordingly, there is a great demand for developing a new method for treating the surface of the elastomers effectively without any problem.